Photoacoustic spectroscopy (PAS) is an analytical method that involves stimulating a sample by light and subsequently detecting sound waves emanating from the sample. Typically, only a narrow range of wavelengths of light are introduced into a sample. Such narrow range of wavelengths of light can be formed by, for example, a laser. Utilization of only a narrow range of wavelengths can enable pre-selected molecular transitions to be selectively stimulated and studied.
A photoacoustic signal can occur as follows. First, light stimulates a molecule within a sample. Such stimulation can include, for example, absorption of the light by the molecule to change an energy state of the molecule. Second, an excited state structure of the stimulated molecule rearranges. During such rearrangement, heat, light, volume changes and other forms of energy can dissipate into an environment surrounding the molecule. Such forms of energy cause expansion or contraction of materials within the environment. As the materials expand or contract, sound waves are generated.
In order to produce a series of sound waves or photoacoustic signals, the light is pulsed or modulated, at a specific resonant acoustic or modulation frequency f. Accordingly, an acoustic detector mounted in acoustic communication with the environment can detect changes occurring as a result of the light stimulation of the absorbing molecule concentration or signal.
Because the amount of absorbed energy is proportional to the concentration of the absorbing molecules, the acoustic signal can be used for concentration measurements.
In typical PAS, a resonant acoustic cavity or sample cell with a quality factor Q is used to isolate and amplify sound wave signals, thereby increasing sensitivity of detection. The light intensity or wavelength is modulated at f. The absorbed energy is accumulated in the acoustic mode of the sample cell during Q oscillation periods. Hence, the acoustic signal is proportional to the effective integration or energy accumulation time t, where t=Q/f. Most often the Q factor is in the range 40–200 and f=1,000–4,000 Hz. For example, if Q=70 and f=1250 Hz, then t=0.056 s
An exemplary prior art apparatus 10 for PAS is shown in FIG. 1. Apparatus 10 comprises a light source 12 configured to emit a beam of radiation into a sample holder 14. Light source 12 can comprise, for example, a laser. Filters (not shown) can be provided between light source 12 and sample holder 14 for attenuating the light prior to its impacting sample holder 14.
Sample holder 14 comprises a sample cell 18 containing a sample 16. Sample cell 18 can comprise a number of materials known to persons of ordinary skill in the art, and preferably comprises a material substantially transparent to the wavelength(s) of light emanating from light source 12. Preferred materials of sample cell 18 will accordingly vary depending on the wavelengths of light utilized in the spectroscopic apparatus.
Sample 16 comprises a material that substantially fills sample cell 18. Such material can be, for example, a fluid such as a liquid or a gas. Sample 16 can, for example, comprise a liquid solution wherein the molecular vibrations that are to be studied are associated with molecules dissolved in the liquid.
Apparatus 10 further comprises an acoustic detector 20 mounted to sample cell 18 and in acoustic communication with sample 16. Acoustic detector 20 can comprise a transducer, such as, for example, a microphone and can be mounted such that a fluid is provided between a surface of detector 20 and sample cell 18. Detector 20 is typically removably mounted to sample cell 18 by, for example, a clamp. Acoustic detector 20 is in electrical communication with an output device 22. Device 22 can be configured to display information obtained from detector 20, and can be further configured to process such information. Output device 22 can comprise, for example, an oscilloscope or a computer.
In operation, a beam of light is generated by source 12 and passed through sample cell 18 to stimulate molecular excitation within sample 16. Non-radioactive decay or molecular rearrangements cause expansions and/or contractions of a material within sample 16 to generate acoustic waves passing from sample 16 through sample cell 18 and to acoustic detector 20. Acoustic detector 20 then detects the acoustic waves and passes signals corresponding to, for example, amplitudes and frequencies of the acoustic waves to output device 22. Output device 22 can be configured to convert information obtained from detector 20 to, for example, a graphical display.
In this way the absorbed laser power is accumulated in the acoustic mode of sample cell 18 for Q acoustic oscillation periods before the sound waves decay. As can be appreciated, the dimensions of sample cell 18 are dependent upon f, where the size of the resonant sample cell cannot be less than half an acoustic wavelength, that is ˜15 cm at f=1,000 Hz. Also, because PAS detectors are sensitive to environmental noise, additional buffer volumes are often added onto the sample cells to suppress background noise.
Achieving longer t's in a fluid-filled sample cell 18 is problematic because of sample cell dimension constraints in addition to intrinsic losses related to gas viscosity and other relaxation processes.
In order to achieve longer energy accumulation times (t's), and therefore better signals, a need exists for methods and apparatus that can detect changes occurring as a result of the light stimulation of an absorbing molecule without the use of a sample cell.